1. Field of the Invention
This invention relates to a method of manufacturing silicon carbide (SiC) based composite materials which comprise silicon carbide reinforced by the dispersion therein of a boride of any element of Groups IVa to VIa of the periodic table, and also relates to a composition of raw materials therefor.
2. Description of the Prior Art
Attempts have been made to use silicon carbide as a material for high temperature engineering components because of its excellent mechanical and chemical properties at an elevated temperature. It is particularly worthy of notice that silicon carbide does not show any reduction of strength even at a temperature of 1500.degree. C. or above, but even tends to have a higher strength than at an ordinary room temperature, if a sintering additive comprising boron or a boron compound is employed, as described in Japanese Patent Publications Nos. 32035/1982 and 34147.1984. It is, therefore, expected to be a good material for gas turbine engines, or other engineering components operating at an elevated temperature of 1400.degree. C. or above.
The greatest drawback of silicon carbide is, however, its low fracture toughness (K.sub.IC value) For example, a hot pressed product of SiC containing boron and a product of SiC made by pressureless sintering and containing boron and carbon have a K.sub.IC value of 2.7 to 2.8 MPa ml.sup.1/2 as determined by the IM (indentation microfracture) method. This value is only a half of the K.sub.IC value of a sintered product of Si.sub.3 N.sub.4 (5 to 7 MPa.multidot.ml.sup.1/2). This is due to the fact that the fracture of a product of SiC made by adding boron proceeds transgranularly and produces only a small surface area, resulting in the consumption of only a small amount of fracture energy. It is known that a SiC-based material obtained by using Al.sub.2 O.sub.3 as a sintering additive has a K.sub.IC value exceeding 6 MPa.multidot.ml.sup.1/2, as its fracture proceeds mainly through the grain boundary (Suzuki: Speech at Fourth Basic Debate on High Temperature Materials, 1984, pages 31 to 34). This material, however, shows a reduction of strength at a high temperature exceeding 1200.degree. C. and has, therefore, only a limited scope of applicability as a material for gas turbine engines, etc.
Attempts have been made to improve the fracture toughness of a SiC-based material by dispersing particles of another substance therein. One of these attempts employs borides of elements of Groups IVa to VIa of the periodic table, e.g. titanium diboride (TiB.sub.2) and zirconium diboride (ZrB.sub.2), as described in Japanese Patent Applications laid open under Nos. 27975/1982, 223272/1984 and 186468/1985, Am. Ceram. Soc. Bull., vol. 66, No. 2, 1987, pages 322 to 324 and 325 to 329, and Journal of the Ceramic Society of Japan, Vol. 93, No. 3, 1985. pages 123 to 129. The SiC-based materials containing particles of borides of these metal elements have not only a high "value of fracture toughness, but also high electrical conductivity. These materials are manufactured by, for example, mixing SiC, boride particles and a sintering additive, molding the mixture and sintering it, or hot pressing it.
The known methods, however, employ a metal boride in powder form and have, therefore, the following drawbacks:
(1) The commercially available powders of borides of Group IVa to VIa elements generally have a large particle diameter and even contain coarse particles having a diameter exceeding 10 microns. The coarse particles are likely to form an origin of fracture and lower the strength of a SiC-based material;
(2) Fine powders of borides of Group IVa to VIa elements are generally so reactive with water that water cannot be used for mixing them with other materials. When the mixture is dried, it is desirable to employ an inert atmosphere. If a slurry prepared by mixing SiC, a fine boride powder and a sintering additive with water is dried in the air to form a powder, the surfaces of the boride particles are oxidized. Therefore, the sintered product shows lower strength at an elevated temperature and poor improvement in fracture toughness. Moreover, the use of these oxidized powders sometimes makes it difficult to obtain a dense sintered product, as a result of swelling or other defects. These problems may be overcome if an organic solvent is used for mixing the materials to form a slurry, and if it is dried in a nonoxidizing atmosphere. This method is, however, likely to present a problem of safety or sanitation and is, moreover, costly, as it requires a spray drier of the explosion-proof type or a vacuum drier. If no liquid is used for mixing the materials, it is difficult to obtain a uniform mixture and the agglomeration of boride particles is likely to result in a product of low strength; and
(3) The powders of borides of Group IVa to VIa elements contain impurities, such as free carbon and oxygen, and the use thereof gives rise to a wide variation of properties of a sintered body, such as high temperature strength and fracture toughness.